The present invention relates to radio range-finding systems and, more particularly, to such systems which utilize field strength measurement to determine the distance from the transmitter to the receiver.
Over the years, a number of techniques have been developed and utilized to measure distance electromagnetically. These include phase, timing, and field strength measurements of radio frequency signals. Phase measurement techniques have been well utilized in navigation systems such as LORAN. Although both phase and time of arrival systems work well for far field measurements, they tend to be relatively complex and therefore costly.
Field strength measurement has been crudely utilized in applications such as "child minding"; a child carries a receiver or transmitter and a cooperating transmitter or receiver is placed at a location near which the child should stay When the distance between the transmitter and receiver exceeds some predetermined value, the field strength at the receiver falls below a predetermined threshold value, and an alarm or some suitable output is actuated. Such field strength based systems are attractive because of their low cost, but they have suffered from poor repeatability and poor accuracy due to the fact that the field strength is influenced by many factors other than distance. Included in these factors are intervening objects, and changing geometry of the transmitting and/or receiving antenna.
It is an object of the present invention to provide a novel and relatively economical radio range finding system based on signal field strength measurement.
It is also an object to provide such a system which exhibits a high level of accuracy and repeatability.
Another object is to provide such a system in which the transmitter and receiver may be fabricated readily from commercially available components and in a relatively miniaturized form.
Still another object is to provide a system which is relatively inexpensive, reliable in performance, and relatively long lived.